Exhaust gas recirculation (EGR) systems may be used in diesel internal combustion engines to reduce NOx emissions. In the combustion chambers, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and decreasing the formation of NOx. Further, EGR systems may include coolers to cool the exhaust gas before it is introduced into an intake manifold of a diesel engine.
One approach to utilize multiple EGR loops is described in U.S. 2005/0028515. In this system, an exhaust control apparatus is applied to an internal combustion engine in which intake sides of a plurality of cylinder groups are connected to a common intake passage and exhaust sides thereof are connected to different exhaust passages. The system includes an exhaust gas flow amount adjusting device for causing a flow amount of exhaust gas, which is discharged through each of the exhaust passages of the cylinder groups, to be changed, an exhaust gas flow amount difference estimating device for estimating a difference in an exhaust gas flow amount between the exhaust passages of the cylinder groups, and an exhaust gas flow amount control device for controlling the exhaust gas flow amount adjusting device so as to reduce the estimated difference in the exhaust gas flow amount.
However, the inventors herein have recognized a disadvantage with such an approach. In particular, at low mass EGR flows and/or low EGR gas temperatures, solids such as soot, lacquer, tar, etc. may build up in the EGR cooler over a period of time. This build up can reduce gas flow, increase pressure drop, and reduces cooling efficiency.
The above issue may be solved by a system for a vehicle having a diesel engine with multiple EGR loops, where a control system is provided to adjust operation of valves in the EGR loops in a first condition where both valves provide EGR flow to the engine, and then at least temporarily increase flow in the first EGR loop and decrease flow in the second EGR loop when EGR flow is decreased to a condition where buildup in one of said coolers is increased. In this way, the buildup in the loop having increased flow can be reduced via the increased flow rate, flow velocity, and/or temperature, and the buildup in the second loop can be reduced by reducing the amount of flow, or possibly reduced by stopping the flow. Further, by coordinating the adjustments in this way, the engine can still receive the desired total amount of EGR flow dictated via the current operating conditions.
In addition, by selective placement of EGR inlets and/or outlets, sufficient mixing of EGR with fresh air may be provided. For example, one or more EGR inlets may be position in an intake manifold upstream of a split of flow to separate cylinder groups or banks.
In an alternative approach, the above issues may be addressed by a method of operating an engine having a first exhaust gas recirculation loop and second exhaust gas recirculation loop, the loops having respectively a first and second valve located therein and a first and second cooler located therein. The method comprises during a first condition, operating with both the first and second valves providing at least some flow; and during a second condition, operating with the first valve providing more flow to the first cooler than during said first condition and the second valve providing less flow to the second cooler than during said first condition.
Note that the above examples may include various modifications. For example, more than two EGR loops may be provided, and the loops may be provided in different relative locations, such as one loop being operated at a higher pressure level than the other due to turbocharger operation.
Further, it may be possible to alternate the operation of the EGR loops during the second condition, for example, to provide even aging and buildup reduction of the loops and engine cylinders, for example. However, if one loop is more prone to build up, that loop can be operated proportionally more often, or longer, with the higher flow.
Further still variations may also be used as described in more detail herein.